The present invention relates to synchrotilt chair controls. In synchrotilt controls, the chair back and the chair seat both tilt, and generally tilt together, but they tilt at different rates. The back tilts at a faster rate so that as one tilts back, he is less likely to have his feet lifted off of the floor by the rising front edge of the chair seat. In contrast, the other two common types of chair controls include one attached to the chair seat only such that the chair and back tilt at the same rate or one attached to the back only such that the back tilts, but the seat doesn't.
Synchrotilt chair controls typically have a stationary member with a resilient biasing means mounted in the stationary member. A chair back support means is usually pivotally mounted on the stationary member and is operably interconnected with the resilient biasing means. A chair seat support means is mounted on the stationary member and is operably connected to the chair back support means for rearward tilting with the chair back support means, but at a different rate with respect to it.
In order to provide for the differential rate of tilting between the chair seat and back, the chair seat support and the chair back support portions of the control must move relative to one another. Yet, they must be interconnected to one another so that the relative movements of the chair seat and back can be coordinated.
One way that prior artisans have achieved this result is to provide a toggel linkage between the rear of the seat support and the rear of the back support. Examples of prior art patents disclosing such a mechanism include the following:
U.S. Pat. Nos. Lie 2,991,125, issued July 4, 1961; Dufton 3,369,840, issued Feb. 20, 1968; Williams 3,402,964, issued Sept. 24, 1968; Lie 3,455,601, issued July 15, 1969; Kerstholt 3,602,537, issued Aug. 31, 1971; and Williams 3,672,721, issued June 27, 1972.
Another alternative employed by prior artisans is to provide a sliding connection between the rear of the seat support and the rear of the back support member. Examples of this approach include U.S. Pat. Nos. Sengpiel 2,447,601, issued Aug. 24, 1948; Moore 3,072,436, issued Jan. 8, 1963; and Pauquete 4,013,257, issued Mar. 22, 1977.
There are two important drawbacks to these prior art arrangements. Perhaps most importantly, the moving toggel linkage or slide between the rear of the seat support member and the rear of the back support member are subjected to tremendous loading forces and accordingly tend to wear out and otherwise operate inefficiently. The tremendous forces imposed on the rear of a chair control, either a seat supporting member or a back upright supporting member or both, are perhaps not totally appreciated by those skilled in the art. Hence, the prior art devices described above have not enjoyed any significant success.
At least one prior artisan has attempted to overcome this difficulty through the use of two separate torsion members in an attempt to distribute the loading forces. Anderson et al, U.S. Pat. No. 3,545,810, issued Dec. 8, 1970. Even so, the loads imposed at the rear of the seat support member and back upright support member are very high. Further, such mechanisms tend to be extremely cumbersome and complicated, as do the prior art mechanisms described above.
Another problem encountered with the arrangements described above is that a user may get the feeling as he leans back that the chair back and seat are separating from one another due to the slight shift between the seat support member and the back support member at the rear thereof. One prior artisan attempted to eliminate this uneasy feeling by pivotally joining the rear of the seat support to the chair back support and providing for sliding movement at the point at which the back support members are pivotally joined to the stationary chair control housing. Ciuffini et al, U.S. Pat. No. 3,240,528, issued Mar. 15, 1966. Unfortunately, that also is a heavily loaded point and accordingly, there may be a tendency for the sliding bearings to stick or wear out.